Just to add some perhaps pertinent information to this thread, as someone who operates Voyagers on a daily basis.
The door egress/passcom and call for aid systems are separate on a Voyager. The passcom & door egress DO create an alarm in the driving cab and brake application and the driver has a means to override them and stop the train in a suitable location and investigate as applicable.
However, there is a separate call for aid system, which when operated, DOES NOT notify the driver - this system is in effect a supplement to the passcom and can be found in toilets and by the disabled traveller locations - the likely reason for this I imagine was to weed out unnecessary passom alarms such as, say, a class 170, where 99.99% of passcom activations are people pressing the passcom to flush the toilet or an inquisitive child being allowed by their parents to press a big red button. I'm sure that when Voyagers were originally introduced these alarms came through to the cab, but the sheer amount of unnecessary level 3 alarms led to the current setup.
So, I believe the reason for one customer facing staff member per set was to manage those calls for aid in particular, as otherwise the driver wouldn't know anything about it. I imagine also in this day of PRM legislation, were a person in a wheelchair left to their own devices having pressed a call for aid and overcarried or not assisted off the train at ther desired destination that there would be some comeback.
It's not really for me to comment on the politics of the situation, but blaming trade unions for everything that people don't like on the railway isn't the answer either. And remember, as staff most of us don't particularly like the situation either.